The present invention is directed to a tool bit for insertion into a tool bit chuck in a manually operated tool used for chiseling and/or percussion drilling and includes an axially extending chucking shank with at least a locking groove closed at its ends extending transversely of the axial projection and at least two rotary entrainment grooves open at the end of the shank inserted into the chuck.
Tool bits for manually operated tools are disclosed in DE-PS 25 51 125 where the chucking shank of such tool bits has one or two locking grooves closed in the axial projection as well as one or two axially extending rotary entrainment grooves open at the end of the shank to be inserted into a chuck. The tool bit chuck into which the tool bit is inserted has one or two radially displaceable locking elements in the form of balls or rollers.
The locking elements cooperate with the locking grooves closed in the axial projection for retaining tool bits within the tool bit chuck. This retention in the axial projection secures a tool bit from falling out and, in addition, fixes the tool bit in the tool bit chuck in the axial projection in such a way that the tool bit can be pulled out of a bore or hole drilled in a structural member. Considerable loads are developed in the axial projection if a jammed tool bit must be pulled out of a bore in a structural member. Since such problems occur only occasionally, in the past little attention has been paid in the design of tool bits to providing an axially locked connection with the chuck.
The transmission of torque from the tool bit chuck is a more significant problem, especially in view of the trend to make manually operated tools more powerful and to use much larger diameter tool bits in such tools. Due to the trend of increasing the torque to be transmitted to the tool bit, wear of the chucking shank can lead to premature failure. Such failure, due to wear of the chucking shank of the tool bit, can take place long before the normal wear in the working region occurs under every day use of such tool bits. This premature failure of the tool bit has a significant effect on the economics of the tool bit.
Using larger rotary entrainment grooves for providing larger flanks for the transmission of torque fails due to the reduction in the cross-section of the chucking shank which results. The weakening of the tool bit due to the reduction in its cross-section leads to premature failure, for instance by fracture in the region of the chucking shank. A tool bit with a chucking shank designed in this disadvantageous manner is disclosed in EP-A-0 355 071.